A classical method for the formation of a synthetic diamond material comprises the conversion of carbon material into a single crystal diamond body at a high temperature and a high pressure. Such a single crystal body can be smoothened on the surface thereof to an extreme extent. Further, such a single crystal diamond has an enhanced mechanical strength and abrasion resistance. However, this method can hardly accomplish the direct film formation of diamond on the surface of the article. Further, this method cannot cover a large area of the surface of the article by the diamond film.
A method for directly forming a diamond film in a large area on the surface of the article by a CVD (chemical vaper deposition) was developed in the 1980's. Since then, this method has been under improvement. In this method, a hydrogen gas and a gas containing carbon are mixed. The mixed gas is then rendered plasma state. This plasma is then brought into contact with the surface of the article to cause the deposition (i.e., nucleus formation) of a large number of diamond seed crystals, i.e., crystal nuclei on the surface of the article. Subsequently, these crystal nuclei are allowed to grow to form a polycrystalline diamond film layer (hereinafter sometimes referred to as "film") coated on the surface of the article. This method is called "CVD" (chemical vapor deposition). The diamond film coated on the surface of the article by this CVD method comprises polycrystallites because it is a product of a formation process which comprises the formation of a large number of diamond crystal nuclei which are each then allowed to grow so that the surface of the article is continuously covered by diamond crystallites.
The diamond crystallites which have covered the surface of the article then each grow perpendicular to the surface of the article. A report (NEW DIAMOND, Vol. 5, No. 15 (1989), Japan) says that when the diamond crystallites are allowed to grow continuously to orient columnar crystals with (100) plane, one of crystal planes thereof nearly in parallel with the surface of the film, only the (100) planes are exposed from the surface of the resulting diamond film, making it possible to obtain a high quality diamond film having an excellent surface smoothness and little non-diamond components therein. In general, however, some diamond crystallites made by the CVD method continue to increase their crystal sizes while growing perpendicular to the film. But, other diamond crystallites made by the CVD method stop their growth in the course of process to yield the expansion of crystal sizes to other diamond crystallites. The diamond film thus coated is generally observed comprising (111) planes and the other planes such as (110) and (100) exposed at the surface thereof in admixture. The diamond film coated by CVD method is made of an aggregate of diamond crystallites and has the following disadvantages. When the thickness of the diamond film increases, the number of diamond crystallites on the surface of the film decreases and the size of diamond crystallites on the surface of the film increases, resulting in raising the surface roughness of the diamond film, though providing the diamond film with a high surface hardness. As mentioned above, when the diamond crystallites are allowed to grow in a uniform the crystalline orientation, i.e., (100) crystal planes thereof being oriented in parallel with the surface of the film, the surface roughness as well as the crystalline size can be controlled. However, since the (100) plane of the diamond has a smaller diamond hardness than other planes such as (111) plane, the article having diamond film layer(s) coated on the surface thereof in such an arrangement that only the (100) planes are exposed to the exterior does not derive the best hardness from the diamond in the application such as cutting tool requiring a high abrasion resistance. This, the article having the diamond film coated in such an orientation that only the (100) planes are exposed to the exterior is unsuitable for the use in working requiring abrasion resistance. The hardness of a diamond on (100) crystal plane and (111) crystal plane are about 5,700 kg/mm.sup.2 and 9,000 kg/mm.sup.2, respectively, by Knoops scale under a load of 500 g. The hardness on (110) crystal plane is intermediate between these values. It is desirable that the article requiring a high abrasion resistance, such as cutting tool, has a diamond film formed on the surface thereof in such an orientation that the (111) planes, which show the greatest hardness among the planes of diamond crystals, are exposed to the exterior as many as possible.
When the diamond film formed by the CVD method is analyzed by Raman spectropy, non-diamond components such as pyrolitic graphite and amorphous carbon are detected besides diamond. When the content of non-diamond components in the diamond film increases, the diamond film softens as a matter of course and hence shows a drop in wear resistance or abrasion resistance or hardness regardless of plane orientation and deteriorates in thermal conductivity or chemical stability.
In the case of an article requiring a high abrasion resistance, particularly a cutting tool, the desired thickness of the diamond film to be coated thereon is at least 20 .mu.m, preferably from 30 to 100 .mu.m. In the case of an abrasion resistant article requiring a prolonged life, the thickness of the diamond film to be coated thereon is occasionally about not less than 1 mm. An article for such a use, particularly a cutting tool, is brought into contact with a work to be cut when used. Thus, the blade of such a cutting tool needs to be smooth. Otherwise, the surface of the work is scratched more than necessary during cutting. This requirement is common to almost all abrasion resistant articles.
When the polycrystalline diamond film is continuously formed to a thickness of not less than 20 .mu.m on the edge surface of such a cutting tool or other articles requiring a high abrasion resistance by the foregoing CVD method in such an arrangement that the (111) planes are exposed to the exterior by at least 50%, a crystal size of each diamond crystallite measured at the surface of the film grows to not less than 10 .mu.m, about half the thickness of the film. There is a clear correlation between the surface roughness of the worked surface of the work and the mean crystal size of the crystallites forming the edge of the cutting tool if the edge which comes into contact with the work to be cut is not smoothly whetted in advance. For example, when the edge with a 20 .mu.m-thick polycrystalline diamond film which comprises diamond crystallites having a mean particle size of about 10 .mu.m at the surface cuts a work, it will leave the work with a surface roughness (Ry) of about 10 to 15 .mu.m. On the other hand, the edge with a 40 .mu.m-thick polycrystalline diamond film which comprises diamond crystallites having a particle size of about 20 .mu.m cuts the work leaving the surface roughness (Ry) as high as about 15 to 25 .mu.m. Accordingly, in order to minimize the surface roughness of the work after working, it has heretofore been necessary that the tool edge having a single crystalline diamond layer thereon with the thickness as high as not less than 20 .mu.m, for example, be smoothened on the surface of the edge by grinding or whetting. This results in a high production cost that gives a trouble.
In the case of such an article requiring a high abrasion resistance, particularly a cutting tool, the foregoing abrasion resistance of the diamond film coated on the edge is important. At the same time, commonly speaking, the diamond coated film must not be damaged by severe cutting load. Otherwise, the damaged cutting edge not only scratches the work to be cut but also disables cutting.
Accordingly, an abrasion resistant article having a polycrystalline diamond film coated thereon by the CVD method must satisfy three requirements at the same time. In other words, such an abrasion resistant article must have a smooth diamond film, that is, its diamond crystallites must have a minimized mean crystal size. The diamond film must maintain its hardness high. Finally, the diamond film is insusceptible to damage such as crack. In these respects, the abrasion resistant article having the polycrystalline diamond film coated thereon by the CVD method leaves much to be improved. The present invention provides improvements in these requirements.
As the related art techniques for the formation of a plurality of polycrystalline diamond film layers on the surface of an article, there are provided U.S. Pat. Nos. 5,114,696 and 5,432,003, JP-A-8-104597 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), and JP-A-6-172087 so far as the present inventors know.
U.S. Pat. No. 5,114,696 and JP-A-8-104597 disclose a method which comprises alternate lamination of two or more layers of (i) diamond-like material or diamond-like carbon and (ii) diamond on the surface of an article. In the plurality of diamond film layers formed by these methods, the diamond-like carbon (DLC) layer which is not diamond is present interposed between these diamond layers. Accordingly, if an abrasion resistant article, e.g., cutting tool is coated with the diamond film layers formed by these methods, there is a disadvantage in that it shows a change in abrasion resistance during the operation when the abrasion of the diamond film layer proceeds to the diamond-like layer which is not diamond, deteriorating the quality of the worked surface of the work.
Further, U.S. Pat. No. 5,432,003 discloses a multilayer film made of a first layer of microcrystalline diamond and a second layer of polycrystalline diamond having a greater thickness than the first layer. However, the multilayer film having a total thickness of about not more than 2 .mu.m in only disclosed. Accordingly, this disclosure gives no suggestions for solving the problems of the conventional abrasion resistant article, e.g., cutting tool having a diamond film layer coated on the surface thereof to a thickness of at least about 20 .mu.m.
JP-A-6-172087 discloses a technique which may be closest to the present invention. JP-A-6-172087 describes an article having a plurality of diamond film layers formed thereon by the CVD method and a process for production thereof. This article is a product of lamination of a plurality of layers each synthesized under different conditions. That is, even when the outermost diamond film layer is cracked, the crack stops at the other adjacent layer having a different kind of diamond layer interposed as an interface layer, making it possible to inhibit the expansion of such a crack damage. JP-A-6-172780 discloses a process for the production of the diamond coated article wherein a difference in diamond-synthesis conditions is made by varying a material gas ratio (ration of hydrogen gas to carbon-containing gas), a pressure of the gas and/or a microwave power for forming plasma. However, unlike the present invention, the concept of which comprises an object of inhibiting the damage of the diamond coated film itself, JP-A-6-172087 discloses only a limited technical concept of forming an interface layer for stopping a further development of the crack. The disclosure of JP-A-6-172087 does not refer to an optimum diamond multilayer which takes into account all requirements such as surface roughness of the diamond layer, anti-damage properties and abrasion resistance, i.e., life against abrasion, which an abrasion resistant article, particularly cutting tool, should have. In some detail, JP-A-6-172087 discloses only one example of drastically different synthesis conditions (Condition A and Condition B) for two different diamond layers, each adjacent layer formed as shown below:
Synthesis condition A: PA0 Synthesis condition B:
Gas pressure: 10 torr
Material gas: CO 80 cc/min+H.sub.2 20 cc/min
Microwave power: 1.5 kw
Substrate temperature: approx. 700.degree. C.
Synthesis time: approx. 10 hr/badge
Gas pressure: 0.1 torr
Material gas: CO 20 cc/min+H.sub.2 80 cc/min
Microwave power: 4 kw
Substrate temperature: approx. 700.degree. C.
Synthesis time: approx. 10 hr/badge
U.S. Pat. No. 5,173,089 discloses the CVD method for producing a polycrystalline diamond tool, wherein molar ratio (B1/A1) of molar density (B1) of a carbon-containing gas to mol density (A1) of hydrogen gas in the material gas in the first depositing step in which the diamond is deposited to a certain thickness more than 12 .mu.m is smaller than molar ratio (B2/A2) or molar density (B2) of the carbon-containing gas to molar density (A2) of hydrogen gas in the material gas in the depositing step (i.e., (B1)/(A)&lt;(B2)/(A2)). However, the object of the above patent is to provide a single layer of a polycrystalline diamond, having a thickness of more than 40 .mu.m on a tool and to produce a cutting tool by brazing the single diamond layer on a substrate. The resultant diamond crystallites in the single layer will have a mean crystal size of more 20 .mu.m. Thus, this method does not disclose a multilayered diamond tool, but a single diamond layer on the tool. Further, this method uses N.sub.2 in the starting material gas.